Mold for molding foamed tube for tire, and method of manufacturing foamed tube for tire

ABSTRACT

A mold for molding a foamed tube for a tire is formed from a first mold and a second mold. Each mold has an annular concave portion for tube molding, and joining faces which are continuous with the annular concave portion and are formed at an inner side and an outer side, in a radial direction, of the annular concave portion. Each of the joining faces of the first mold and the joining faces of the second mold has horizontal joining faces provided adjacent to the annular concave portion in a direction orthogonal to an axial direction at a radially inner side and a radially outer side of the annular concave portion, and an inclined joining face provided continuously with the horizontal joining face so as to be inclined with respect to the horizontal joining face at one of an inner side and an outer side in the radial direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mold for molding a foamed tube for atire which is used in place of a hollow rubber tube of a pneumatic tire,and to a method of manufacturing a foamed tube for a tire.

2. Description of the Related Art

For example, in order to prevent bursting of tires mounted tomotorcycles for motocross, there are cases in which a donut-shapedtoroidal sponge, which is formed from an elastic body of rubber or thelike, is filled in the tire in place of an air tube.

Conventionally, such a toroidal sponge is manufactured as follows.

(1) First, a rubber composition containing foaming agents, foamingauxiliaries and the like, is extruded at an extruder so as to form asolid cylindrical rubber material. Both ends thereof are joined togetherand a donut-shaped torus 14 such as shown in FIG. 2 is molded.

(2) Next, by using a mold 102 formed from an upper die 102A and a lowerdie 102B such as shown in FIG. 12A, the donut-shaped torus 14 is filledinto the mold 102 as shown in FIG. 12A. After the upper die 102A and thelower die 102B are fixed by an unillustrated fastening hardware, themold 102 is sandwiched between heating plates and vulcanization iscarried out.

In the conventional mold 102, a joining face 103A of the upper die 102Aand a joining face 103B of the lower die 102B are both horizontal.

After vulcanization, when the mold 102 is removed from the heatingplates and the fastening hardware is undone, the donut-shaped torus 14expands, and a sponge-like foamed tube for a tire is obtained.

The foamed tube for a tire which is obtained in this way is coolednaturally at room temperature, and is kept until the foaming ratedecreases and is stable. A foamed tube for a tire whose dimensions havestabilized is then served for use (i.e., actually mounted to amotorcycle or the like).

However, after vulcanization, when the mold is opened (i.e., when themold is opened to the atmosphere), the gas generated in the rubberbecomes air bubbles and expands. The rubber composition suddenly expands(the volume thereof suddenly increases to about 500 to 1200%)substantially simultaneously with the opening of the mold 102. Theswelled rubber composition enters into the narrow gap between thejoining faces of the upper die 102A and the lower die 102B which areopened slightly, and bursts out to the exterior of the mold 102.

At this point in time, the expanding of the rubber composition is local,and the elongation of the outer surface at this swelled portion isextremely high. Thus, as shown in FIG. 13, tears 104, 106 arise in thesurface of the rubber composition which has burst out and swelled. Asshown in FIG. 14, a problem arises in that there are many cases in whichthe tears 104, 106 remain in the foamed tube for a tire (thedonut-shaped torus 14) which is finally obtained. Thus, conventionally,the speed of opening the vulcanizing press to which the mold 102 ismounted is made to be extremely fast in order to handle the expanding ofthe rubber composition, or the joining face 103A and the joining face103B of the upper die 102A and the lower die 102B are made to beextremely wide so as to suppress the bursting out of the rubbercomposition to the exterior of the mold, or the foaming rate is set tobe low to the extent that tears do not arise, or the blendingcomposition of the rubber composition is adjusted so as to make themodulus high.

However, making the opening speed of the mold 102 be as fast as thespeed at which the rubber composition expands results in the vulcanizingpress becoming a large-sized equipment. The cost thereof then increasesdrastically, and a large space is required for the placement thereof.

On the other hand, making the horizontal mating surfaces of the mold 102wide leads to the outer dimensions of the vulcanization mold becominggreater. The cost of the mold 102 then inevitably increases, thehandling thereof is difficult and workability is poor.

Further, in order to address the above problem by adjusting the blendingcomposition of the rubber composition, the properties of the rubbercomposition have to be restricted, and there are cases in which desiredproduct properties cannot be obtained.

SUMMARY OF THE INVENTION

In view of the aforementioned, an object of the present invention is toprovide a mold for molding a foamed tube for a tire and a method ofmanufacturing a foamed tube for a tire which can overcome theabove-described problems of the conventional art.

In order to achieve the above object, a first aspect of the presentinvention is a mold for molding an annular foamed tube for a tire,comprising: a first mold having a first annular concave portion whichmolds one side, in an axial direction, of the foamed tube for a tire;and a second mold having a second annular concave portion which moldsanother side, in the axial direction, of the foamed tube for a tire,wherein when the first mold and the second mold are combined in theaxial direction, the first annular concave portion and the secondannular concave portion form an annular space for molding the foamedtube for a tire, the first mold has first joining faces which areconnected to the first annular concave portion and which are positionedat an inner side and an outer side, in a radial direction, of the firstannular concave portion, and the first joining faces include horizontaljoining faces provided adjacent to the first annular concave portion ina direction orthogonal to the axial direction at the inner side and theouter side, in the radial direction, of the first annular concaveportion, and inclined joining faces each provided continuously with arespective one of the horizontal joining faces so as to be inclined withrespect to the horizontal joining face at one of an inner side and anouter side in the radial direction, the second mold has second joiningfaces which are connected to the second annular concave portion andwhich are positioned at an inner side and an outer side, in the radialdirection, of the second annular concave portion, and the second joiningfaces include horizontal joining faces provided adjacent to the secondannular concave portion in a direction orthogonal to the axial directionat the inner side and the outer side, in the radial direction, of thesecond annular concave portion, and inclined joining faces each providedcontinuously with a respective one of the horizontal joining faces so asto be inclined with respect to the horizontal joining face at one of aninner side and an outer side in the radial direction, and the first moldand the second mold have configurations which complementarily completeone another via the first joining faces and the second joining faces.

Operation of the mold for molding a foamed tube for a tire based on theabove-described first aspect will now be explained.

First, the foaming-agent-containing rubber composition, which is formedin a toroidal configuration, is positioned in the first annular concaveportion (or the second annular concave portion) of the mold which hasbeen preheated, and the first mold and the second mold are closed.

Thereafter, the molds are heated by a heater, and vulcanization of therubber composition is carried out.

In this way, gas is generated by the foaming agent, innumerable,independent air bubbles form in the rubber composition, and the internalpressure of the rubber composition rises.

After heating has been carried out for a predetermined period of time,the molds are opened, and the rubber composition is removed.

When the molds are opened, the air bubbles expand, and the rubbercomposition swells and becomes a foamed rubber.

In the molds, each of the joining faces of the first mold and thejoining faces of the second mold has horizontal joining faces providedadjacent to the annular concave portion in a direction orthogonal to anaxial direction at an inner side and an outer side, in the radialdirection, of the annular concave portion, and an inclined joining faceprovided continuously with each horizontal joining face so as to beinclined with respect to the horizontal joining face at one of an innerside and an outer side in the radial direction. Accordingly, in theinitial stages of opening the two molds which have been combined, thegap between the inclined joining faces of the first mold and theinclined joining faces of the second mold remains extremely small ascompared with the dimension by which the first mold and the second moldare separated.

When the molds are opened immediately after vulcanization molding of therubber composition has been completed, substantially simultaneously, therubber composition swells rapidly, and first, the rubber compositionenters into a gap between the horizontal joining faces of the first moldand the horizontal joining faces of the second mold. The foamed rubbercomposition which has entered into the gap between the horizontaljoining face portions abuts the inclined joining faces at the outersides and stops.

Immediately after the molds are opened, as mentioned above, the gapbetween the inclined joining face portions remains extremely narrow.Further, the direction of the gap between these inclined joining faceportions is much different than the direction of swelling of the rubbercomposition which has entered into the gap between the horizontaljoining face portions. Moreover, as the mold is opened, even if the gapbetween the horizontal joining faces becomes large, the size of theswelling rubber composition at the gap between the horizontal joiningface portions simultaneously becomes large. Accordingly, the force ofthe swelling rubber composition entering into the gap between theinclined joining faces is small.

Then, until the molds are completely opened, the swelled, foamed rubbercomposition is pushed by its own swelling force against the inclinedjoining faces, and sliding resistance is generated. Thus, the foamedrubber composition does not move within the molds. Namely, until themolds are completely opened, the swelled, foamed rubber composition doesnot protrude out to the exterior of the molds from the gap between theinclined joining faces.

In this way, because sudden swelling of the distal end portion of therubber composition, i.e., elongation of the surface thereof, can besuppressed, tears do not form in the surface of the swelled rubbercomposition.

The molds are then completely opened, and the foamed, toroidal rubbercomposition is removed therefrom and left at room temperature until thefoaming rate is stabilized. The production of the foamed tube for a tireis thereby completed.

In a second aspect of the present invention, of the inclined joiningfaces of the first joining faces and the inclined joining faces of thesecond joining faces, at least the inclined joining faces which arepositioned at the outer side in the radial direction form an angle of 90to 120° with respect to the horizontal joining face adjacent thereto.

Next, operation of the mold for molding a foamed tube for a tire basedon the above-described second aspect will be explained.

If the angle of the inclined joining face positioned at the outer sidein the radial direction, with respect to the horizontal joining faceadjacent thereto, is 120° or more, the outer shape of the mold becomestoo large, which is not effective.

On the other hand, if the angle of the inclined joining face withrespect to the horizontal inclined surface is less than 90°, it is notpossible to open and close the first mold and the second mold unless atleast one of the first mold and the second mold is divided into pluralpieces.

If the angle between the inclined joining face and the horizontaljoining face of the first mold and the angle between the inclinedjoining face and the horizontal joining face of the second mold aredifferent, opening and closing of the first mold and the second mold arepossible. However, even if such a structure is employed, if the inclinedjoining face positioned at the outer side in the radial direction is 90°or less with respect to the horizontal joining face, the foamed rubbercomposition catches on the mold and removal thereof will be difficult.

Note that the angle of the inclined joining face positioned at the outerside in the radial direction, with respect to the horizontal joiningface adjacent thereto, is preferably 93 to 105°.

In the third aspect of the present invention, a cross-sectionalconfiguration, along the axial direction, of the annular space iscircular, and given that a radius of the annular space is R and aprojected length, in the axial direction, of the inclined joining faceis H, a relation R≦H≦3R is satisfied.

Next, operation of the mold for molding a foamed tube for a tire basedon the above-described third aspect will be explained.

If R>H, there is the concern that, when the foaming rate is 6 times ormore, cracks will form in the foamed rubber composition.

If H>3R, no further improvement in the effect of preventing generationof cracks in the foamed rubber composition is discerned (the effectshave already topped out), and the mold becomes large-sized.

Note that it is even more preferable that the relation 2.0R ≦H≦2.5R issatisfied.

Further, if the cross-sectional configuration of the annular space is aconfiguration other than circular (e.g., is oval), it is preferable todetermine the radius of a circle set to have the same cross-sectionalarea, and to determine the dimensions of the respective portions suchthat the relationship between the determined radius and the length Hsatisfies the above stipulation.

A fourth aspect of the present invention is a method of manufacturing afoamed tube for a tire, comprising the steps of: (a) filling afoaming-agent-containing rubber composition, which generates gas whenheated, into at least the annular space of the mold recited in claim 1;and (b) heating and vulcanizing the foaming-agent-containing rubbercomposition.

The method of manufacturing a foamed tube for a tire based on theabove-described fourth aspect has the same contents as the operation ofthe first aspect, and therefore, description thereof will be omitted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side view showing a vicinity of a cut surface of anelongated extruded rubber composition used in the device and method ofthe present invention, and FIG. 1B is a side view showing a joinedportion of the elongated extruded rubber composition of FIG. 1A.

FIG. 2 is a perspective view of a donut-shaped torus in which theelongated extruded rubber composition is joined.

FIG. 3A is a cross-sectional view of a mold of the present invention inan open state, and FIG. 3B is a cross-sectional view of the mold of thepresent invention in a closed state.

FIG. 4 is a partial, enlarged cross-sectional view of the mold which isclosed.

FIG. 5 is a front view of main portions of a press machine to which themold is mounted.

FIGS. 6A through 6D are explanatory diagrams showing processes ofopening the mold.

FIG. 7 is a perspective view of a foamed tube for a tire which ismanufactured on the basis of the present invention.

FIG. 8 is a partial, enlarged cross-sectional view of a mold relating toanother embodiment.

FIG. 9 is a partial, enlarged cross-sectional view of a mold relating toyet another embodiment.

FIG. 10 is a partial, enlarged cross-sectional view of a mold relatingto still another embodiment.

FIG. 11 is a partial, enlarged cross-sectional view of a mold relatingto still yet another embodiment.

FIG. 12A is a cross-sectional view of a conventional mold in an openstate, and FIG. 12B is a cross-sectional view of the conventional moldin a closed state.

FIG. 13 is a cross-sectional view of the conventional mold in a state ofbeing opened slightly.

FIG. 14 is a cross-sectional view of the conventional mold, and a foamedtube for a tire which is manufactured by the mold and in which tearshave arisen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described with referenceto the drawings.

In the present embodiment, explanation will be given of a method ofmanufacturing a foamed tube for a tire which is used in a pneumatic tireof a motorcycle.

First, a rubber composition containing a foaming agent is extruded fromthe die of an extruder, which die has a circular opening, so as toobtain an elongated extruded rubber composition whose cross-sectionalconfiguration orthogonal to the longitudinal direction thereof iscircular. By using the extruder, it is possible to continuously obtain ahomogeneous elongated extruded rubber composition whose cross-sectionalconfiguration is constant and whose dimensions are constant.

The rubber composition containing a foaming agent is formed of, forexample, 100 parts by weight of butyl rubber, 20 to 50 parts by weightof a filler, 0 to 20 parts by weight of process oil, 1 to 5 parts byweight of a vulcanizing agent, 1 to 3 parts by weight of a vulcanizationaccelerator, 4 to 15 parts by weight of a foaming agent, 0 to 8 parts byweight of a foaming auxiliary, and the like.

Dinitrosopentamethylenetetraamine (DPT), azodicarbonamide (ADCA),dinitrosopentastyrenetetramine or benzenesulfonyl hydrazide derivatives,oxybisbenzenesulfonyl hydrazide (OBSH) or the like can be used as thefoaming agent.

An auxiliary agent which is generally used in the manufacture of afoamed product, such as urea, zinc stearate, zinc benzene sulfinate,zinc white or the like, is preferably used as the foaming auxiliary.

Substances other than those listed above may be used as the foamingagent and the foaming auxiliary.

Next, an elongated extruded rubber composition 10, which has beenextruded, is cut into a predetermined length by a cutter or the like.

As shown in FIG. 1A, it is preferable that a cut surface 12 of theelongated extruded rubber composition 10 is formed at an incline, ratherthan at a right angle, with respect to the longitudinal direction. Anangle θ of the cut surface 12 shown in FIG. 1A is preferably about 30°.In this way, the surface area of the cut surface, i.e., the surface areaof joining, can be made large.

Next, as shown in FIG. 1B, the cut surfaces 12 of both ends are joinedtogether, such that the elongated extruded rubber composition 10 becomesa donut-shaped torus 14 as shown in FIG. 2.

Because the slantly cut surfaces 12, which are clean, newly cut anduniform without curling wrinkles or curling stripes, are joinedtogether, the strength of a joint portion 15 can be increased to nearthat of the other portions (the non-joint portions). Entry of air andseparation at the joint after vulcanization can be prevented.

The donut-shaped torus 14 formed in this way is vulcanized in a mold 16which will be described hereinafter.

(Manufacture of Mold)

As shown in FIG. 3A, the mold 16 is formed from an upper die 16A servingas a first mold and a lower die 16B serving as a second mold, which canbe opened and closed in the vertical direction. An annular concaveportion 18A, whose cross-section is semicircular, is formed in the upperdie 16A, and an annular concave portion 18B, whose cross-section issemicircular, is formed in the lower die 16B such that, when the upperdie 16A and the lower die 16B are closed, a donut-shaped, toroidal space18, which serves as an annular space whose cross-sectional configurationis circular, is formed in the interior (refer to FIGS. 3B and 4; adiameter DA of a central portion is 980 mm, and a radius R is 40 mm).

As shown in FIG. 3A, in the upper die 16A, an inner side mating face 24is formed at the inner side, in the radial direction, of the annularconcave portion 18A, and an outer side mating face 25 is formed at theouter side in the radial direction.

The inner side mating face 24 has a horizontal joining face 24A, and aninclined joining face 24B which is connected to the inner end, in theradial direction, of the horizontal joining face 24A.

As shown in FIG. 4, an angle α₁ of the inclined joining face 24B withrespect to the horizontal joining face 24A is preferably in a range of90° to 120°, and is more preferably in a range of 93° to 105°. In thepresent embodiment, the angle α₁ is set to be 95°.

A vertical direction dimension H₁ of the inclined joining face 24B ispreferably in a range of 1 to 3 times a radius R of the space 18, and ismore preferably in a range of 2.0 to 2.5 times. In the presentembodiment, the dimension H₁ is set to be 1.25 times the radius R.

Further, a width W₁, in the radial direction, of the horizontal joiningface 24A is preferably in a range of 5 to 30 mm. In the presentembodiment, the width W₁ is set to be 15 mm.

On the other hand, as shown in FIG. 3A, the outer side mating face 25has a horizontal joining face 25A, and an inclined joining face 25Bwhich is connected to the outer side end, in the radial direction, ofthe horizontal joining face 25A.

As shown in FIG. 4, an angle α₂ of the inclined joining face 25B withrespect to the horizontal joining face 25A is preferably in a range of90° to 120°, and is more preferably in a range of 93° to 105°. In thepresent embodiment, the angle α₂ is set to be 95°.

A vertical direction dimension H₂ of the inclined joining face 25B ispreferably in a range of 1 to 3 times the radius R of the space 18, andis more preferably in a range of 2.0 to 2.5 times. In the presentembodiment, the dimension H₂ is set to be 1.25 times the radius R.

Further, a width W₂, in the radial direction, of the horizontal joiningface 25A is preferably in a range of 5 to 30 mm. In the presentembodiment, the width W₂ is set to be 15 mm.

As shown in FIG. 3A, in the lower die 16B, an inner side joining face 26is formed at the inner side, in the radial direction, of the annularconcave portion 18B, and an outer side joining face 27 is formed at theouter side in the radial direction.

The inner side joining face 26 has a horizontal joining face 26A, and aninclined joining face 26B which is connected to the inner end, in theradial direction, of the horizontal joining face 26A.

In the present embodiment, as shown in FIGS. 3B and 4, when the upperdie 16A and the lower die 16B are closed, the horizontal joining face26A of the lower die 16B fits tightly to the horizontal joining face 24Aof the upper die 16A, the inclined joining face 26B of the lower die 16Bfits tightly to the inclined joining face 24B of the upper die 16A, thehorizontal joining face 27A of the lower die 16B fits tightly to thehorizontal joining face 25A of the upper die 16A, and the inclinedjoining face 27B of the lower die 16B fits tightly to the inclinedjoining face 25B of the upper die 16A.

Next, a press machine 28 to which the mold 16 is mounted will bedescribed in accordance with FIG. 5. Note that, although usually athrust-up mold is used in which a piston rod is disposed at the lowerside of the mold 16, here, explanation will be given of a push-downmold.

Heating plates 34, which have in the interiors thereof heat sources (notshown) such as heaters or the like, are mounted to a base 30 of thepress machine 28 and to the lower end of a piston rod 32 which isdisposed above the base 30 and moves up and down. The upper die 16A isfixed to the upper heating plate 34 and the lower die 16B is fixed tothe lower heating plate 34 by mounting hardware (not shown)respectively.

Note that, in the press machine 28, the moving speed of the piston rod32 can be changed freely.

(Operation)

Next, the method of vulcanizing the donut-shaped torus 14 will bedescribed.

The mold 16 is preheated, the donut-shaped torus 14 is placed in theannular concave portion 18B of the lower die 16B, the piston rod 32 islowered, and the upper die 16A and the lower die 16B are fit tightlytogether (refer to FIG. 3B, and note that the donut-shaped torus 14 isnot shown in FIG. 3B).

Thereafter, the donut-shaped torus 14 is heated for a predeterminedperiod of time at a predetermined temperature set in advance, and isvulcanized.

When vulcanization is carried out, gas is generated within the rubber ofthe donut-shaped torus 14 due to the foaming agent, and the donut-shapedtorus 14 swells within the space 18.

After the predetermined period of time has elapsed, the piston rod 32 israised, and the mold 16 is opened.

When the mold 16 is opened, the air bubbles of gas within the rubberswell, and the rubber composition foams.

In the mold 16, the outer sides of the horizontal joining face 24A andthe horizontal joining face 25A are the inclined joining face 24B andthe inclined joining face 25B, and the outer sides of the horizontaljoining face 26A and the horizontal joining face 27A are the inclinedjoining face 26B and the inclined joining face 27B. Thus, as shown inFIG. 6A, in the initial stage of opening, a gap S_(B) between theinclined joining face portions remains extremely narrow with respect toa separation dimension S_(A) between the upper die 16A and the lower die16B. Further, the bulging portion of the foamed rubber composition ispressed by the inclined joining faces 24B, 25B of the upper die 16A, sothat the foamed rubber composition does not suddenly bulge out directlyinto the atmosphere from the region between the horizontal matingsurfaces.

Note that the pushing-out of the foamed rubber composition to theexterior of the mold is suppressed until the mold 16 is completelyopened and the foamed rubber composition is removed therefrom (refer toFIGS. 6B and 6C).

As shown in FIG. 6D, when the mold 16 is completely opened and thedonut-shaped torus 14 is removed from the mold 16, the donut-shapedtorus 14 swells further, and becomes a form having a circularcross-section and no cracks.

Thereafter, the donut-shaped torus 14 is left at room temperature untilthe foaming rate stabilizes, whereby the production of a foamed tube 44for a tire which has absolutely no cracks, such as shown in FIG. 7, iscompleted.

EXPERIMENTAL EXAMPLE

In order to confirm the effects of the present invention, vulcanizationmolding of a foamed tube for a tire was carried out in both aconventional mold and in the mold of the present invention.

The conventional mold was the mold shown in FIG. 12. A radius R of thespace was 40 mm, and the foaming rate was 800%. Generation of tears inthe foamed tube for a tire when the mold was opened after vulcanizationmolding could not be prevented even at a high mold opening speed (200mm/sec). However, in accordance with the mold 16 of the embodiment (α₁,α₂=95°, W₁, W₂=15 mm, H₁, H₂=50 mm), a foamed tube for a tire, whoseexternal appearance of the surface was good and in which there were notears even with a low opening speed (120 mm/sec), was obtained.

Further, in accordance with the mold 16 of the embodiment, when theopening speed of the mold 16 was 200 mm/sec, it was possible to increasethe foaming rate to 1400%.

Note that, given that the volume before vulcanization is V₀ (cm³) andthe volume after vulcanization and swelling is V₁ (cm³), the foamingrate V_(s) is expressed as V_(s)=V₁/V₀×100 (%).

Other Embodiments

In the mold 16 of the above-described embodiment, the inclined joiningface 24B and the inclined joining face 26B are fit tightly togetheralong the entire surfaces thereof, and the inclined joining face 25B andthe inclined joining face 27B are fit tightly together along the entiresurfaces thereof. However, the present invention is not limited to thesame, and as shown in FIGS. 8 through 11, a gap 36 may be formed at aportion between the inclined mating surfaces.

Further, the inclined mating surface 24B and the inclined mating surface26B, or the inclined mating surface 25B and the inclined mating surface27B, function as guides at the time of aligning the upper die 16A andthe lower die 16B. However, if the horizontal surfaces are fit closelyto one another and other portions have an aligning function, the narrowgap 36 may be formed at all portions between the inclined joining face24B and the inclined joining face 26B, and between the inclined joiningface 25B and the inclined joining face 27B.

As described above, in accordance with the mold for molding a foamedtube for a tire and the method of manufacturing a foamed tube for a tireof the present invention, an excellent effect is achieved in that it ispossible to manufacture a foamed tube for a tire which has no tears.

1. A method of manufacturing a foamed tube for a tire, comprising thesteps of: (a) filling a foaming-agent-containing rubber composition,which generates gas when heated, into at least the annular space of amold comprising: a first mold having a first annular concave portionwhich molds one side, in an axial direction, of the foamed tube for atire; and a second mold having a second annular concave portion whichmolds another side, in the axial direction, of the foamed tube for atire, wherein when the first mold and the second mold are combined inthe axial direction, the first annular concave portion and the secondannular concave portion form an annular space for molding the foamedtube for a tire, the first mold has first joining faces which areconnected to the first annular concave portion and which are positionedat an inner side and an outer side, in a radial direction, of the firstannular concave portion, and the first joining faces include horizontaljoining faces provided adjacent to the first annular concave portion ina direction orthogonal to the axial direction at the inner side and theouter side, in the radial direction, of the first annular concaveportion, and inclined joining faces each provided continuously with arespective one of the horizontal joining faces so as to be inclined withrespect to the horizontal joining face at one of an inner side and anouter side in the radial direction, the second mold has second joiningfaces which are connected to the second annular concave portion andwhich are positioned at an inner side and an outer side, in the radialdirection, of the second annular concave portion, and the second joiningfaces include horizontal joining faces provided adjacent to the secondannular concave portion in a direction orthogonal to the axial directionat the inner side and the outer side, in the radial direction, of thesecond annular concave portion, and inclined joining faces each providedcontinuously with a respective one of the horizontal joining faces so asto be inclined with respect to the horizontal joining face at one of aninner side and an outer side in the radial direction, and the first moldand the second mold have configurations which complementarily completeone another via the first joining faces and the second joining faces;and (b) heating and vulcanizing the foaming-agent-containing rubbercomposition.
 2. A method of manufacturing a foamed tube for a tire,comprising the steps of: (a) preheating one of a first mold and a secondmold, the first mold having a first annular concave portion which moldsone side, in an axial direction, of the foamed tube for a tire, thesecond mold having a second annular concave portion which molds anotherside, in the axial direction, of the foamed tube for a tire, whereinwhen the first mold and the second mold are combined in the axialdirection, the first annular concave portion and the second annularconcave portion form an annular space for molding the foamed tube for atire, the first mold has first joining faces which are connected to thefirst annular concave portion and which are positioned at an inner sideand an outer side, in a radial direction, of the first annular concaveportion, and the first joining faces include horizontal joining facesprovided adjacent to the first annular concave portion in a directionorthogonal to the axial direction at the inner side and the outer side,in the radial direction, of the first annular concave portion, andinclined joining faces each provided continuously with a respective oneof the horizontal joining faces so as to be inclined with respect to thehorizontal joining face at one of an inner side and an outer side in theradial direction, the second mold has second joining faces which areconnected to the second annular concave portion and which are positionedat an inner side and an outer side, in the radial direction, of thesecond annular concave portion, and the second joining faces includehorizontal joining faces provided adjacent to the second annular concaveportion in a direction orthogonal to the axial direction at the innerside and the outer side, in the radial direction, of the second annularconcave portion, and inclined joining faces each provided continuouslywith a respective one of the horizontal joining faces so as to beinclined with respect to the horizontal joining face at one of an innerside and an outer side in the radial direction, and the first mold andthe second mold have configurations which complementarily complete oneanother via the first joining faces and the second joining faces; (b)positioning and placing, within the annular concave portion of the moldwhich is preheated, a foaming-agent-containing rubber composition whichgenerates gas when heated; (c) combining and closing the first mold andthe second mold; (d) heating the molds and carrying out vulcanization ofthe rubber composition; and (e) after carrying out heating for apredetermined period of time, opening the molds and removing the rubbercomposition.
 3. A method of manufacturing a foamed tube for a tireaccording to claim 2, wherein said step (c) of combining and closing thefirst mold and the second mold includes a step of airtightly joining thehorizontal joining faces of the first joining faces and the horizontaljoining faces of the second joining faces when closing the molds.
 4. Amethod of manufacturing a foamed tube for a tire according to claim 3,wherein said step (d) of heating the molds and carrying outvulcanization of the rubber composition includes a step in which gas isgenerated by the foaming agent, innumerable independent air bubbles areformed in the rubber composition, and an internal pressure of the rubbercomposition rises.
 5. A method of manufacturing a foamed tube for a tireaccording to claim 4, wherein said step (e) of, after carrying outheating for a predetermined period of time, opening the molds andremoving the rubber composition, includes: a step in which the rubbercomposition swells suddenly substantially simultaneously with opening ofthe molds, and first, the rubber composition enters into a gap betweenthe horizontal joining faces of the first mold and the horizontaljoining faces of the second mold; a step in which the foamed rubbercomposition which has entered into the gap between the horizontaljoining faces abuts the inclined joining faces at the outer sides andstops; and a step in which the swelled, foamed rubber composition ispushed by its own swelling force against the inclined joining faces andremains within the molds until the molds are completely opened, andtherefore, the rubber composition can be prevented from protruding outto an exterior of the molds before the molds are completely opened.